AFOQT Practice Test

What defines a galaxy?

1. A single star surrounded by planets

2. A collection of stars, dust, and gas bound together by gravity

3. A type of celestial body that emits light

4. Group of planets orbiting a sun

The correct answer is: A collection of stars, dust, and gas bound together by gravity

The definition of a galaxy is accurately captured by the concept of a collection of stars, dust, and gas bound together by gravity. Galaxies can vary in size and structure, containing millions to trillions of stars, along with interstellar matter. The gravitational pull among these elements allows them to form a cohesive unit, distinguishing galaxies from other astronomical structures that might consist of fewer components or lack the gravitational binding. Options like a single star surrounded by planets or a group of planets orbiting a sun represent more specific celestial formations that are part of a galaxy but do not encompass the broader definition. Similarly, the description of a celestial body that emits light may pertain to stars, which are indeed found within galaxies but again does not convey the comprehensive nature of what defines a galaxy. The concept of gravitational binding among the various components is fundamental to understanding how galaxies exist and interact in the universe.